This disclosure relates generally to control systems and methods for an electrophotographic printing machine and, more particularly, concerns systems and methods for registering different color component images of a multi-color image.
In a typical electrophotographic printing process, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface of the photoconductive member. The charged portion of the photoconductive member is exposed to a light image of an original document being reproduced. Exposure of the charged photoconductive member selectively dissipates the charges in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material includes toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules to the latent image so as to form a toner powder image on the photoconductive member. The toner powder image is then transferred from the photoconductive member to a copy sheet. The toner particles are heated to permanently affix the powder image to the copy sheet.
The foregoing generally describes a typical black and white electrophotographic printing machine. With the advent of multicolor electrophotography, it is desirable to use an architecture that includes a plurality of image forming stations. One example of the plural image forming station architecture utilizes an image-on-image (IOI) system in which a photoreceptive member (photoreceptor) is recharged, reimaged and developed for each color separation (each color component of the multi-color image). This charging, imaging, developing and recharging, reimaging and developing, all followed by transfer to paper, is done in a single revolution of the photoreceptor in so-called single-pass machines, while multipass architectures form each color separation with a single charge, image and develop, with separate transfer operations for each color.
In single-pass color machines and other high speed printers, it is desirable to utilize as much of the surface area of the photoreceptor as possible to improve the efficiency and print speed of the printer. The photoreceptor typically is a belt that has a seam which is an area of the photoreceptor that is unusable for forming images thereon. A standard way of marking the seam is to have a hole located at a known distance therefrom and to trigger image formation from that hole. Many print jobs, however, vary in the size of media used, and it is therefore desirable to utilize the photoreceptor in what is known as a variable pitch mode. It is further desirable to utilize this variable pitch mode without having to change the photoreceptor belt to vary the pitch number for the particular print job.
A control system and method for controlling an imaging device in a single-pass multi-color electrophotographic printing machine has been described in U.S. Pat. No. 6,181,887, the disclosure of which is incorporated herein by reference in its entirety. In such electrophotographic printing machines, a belt hole sensor detects the passage of a timing aperture that identifies the location of the seam in a photoreceptor belt. After identifying the location of the seam, a controller calculates image areas where images can be placed on the belt by sequentially-placed imaging stations so as to align images placed by different imaging stations, and to avoid the placement of an image on the seam. In U.S. Pat. No. 6,181,887, the images are formed in relation to a series of “virtual holes” representing positions along the length of the belt. The movement of the belt corresponds with (and is tracked by) an encoder that provides a series of regularly spaced encoder counts or signals indicating the incremental movement of the belt. For example, the encoder is disposed on a roller about which the photoreceptor belt is mounted, and as the roller rotates due to belt movement, the encoder outputs pulses defining the encoder signal. After the belt has moved a predetermined distance, a signal is provided to the appropriate imaging station to cause the imaging station to form an image on the belt.
Typical in such systems and methods, when the timing aperture is detected by the belt hole sensor, the next output encoder count is established as the reference point for the belt seam. Each virtual belt hole is based on this reference point, with the placement of the virtual belt holes in relation to the belt determined from the number of encoder counts output after the reference point encoder count. After a specific number of encoder counts are output, the first virtual hole is representatively identified on the belt and a first image initiation signal is provided by the controller to the first imaging station to initiate the formation of an image containing the first color component by that first imaging station on the belt. After the output of an additional number of encoder counts, the first virtual hole is deemed to be positioned near the second imaging station, and therefore a second image initiation signal is provided to the second imaging station by the controller to cause the formation of an image containing the second color component by the second imaging station on the belt. This process is repeated for the remaining imaging stations until each imaging station forms its color component image juxtaposed with the color component image of the other stations for a particular image. The multicolor toner image then is transferred to the copy sheet and fused.
The aforementioned system and method, however, are limited because the encoder is not synchronous with the imaging stations. In the aforementioned systems and methods, the motors in the imaging stations are controlled through the use of a Master Clock providing a Master Clock signal. The Master Clock signal is not synchronous with the encoder signal corresponding to the movement of the photoreceptor belt. As described above, the image initiation signals sent to the imaging stations are based on the encoder signal. Accordingly, the imaging stations receive image initiation signals at times that are not synchronized with the signal from the Master Clock. Thus, when an imaging station receives an image initiation signal, it must compensate for the asynchronicity between that signal and the clock signal, thus producing a delay between receipt of the image initiation signal and the actual initiation of the image formation process by the imaging station. A delay occurs with each imaging station, but to different degrees because the image initiation signals are received by the imaging stations at varying times between consecutive clock counts.
The aforementioned delays detrimentally affect the operation of electrophotographic printing machines. For each imaging station, the delay causes error in the precise location of the image at the point on the belt corresponding to a virtual hole.
To compensate for the aforementioned delay, some systems have modified the operation of the imaging device to more closely coincide with the image initiation signal. This modification is generally done by using reference marks or chevrons just before the image areas on the photoreceptor belt and by the speeding up and slowing down of each of the motors driving the imaging stations to cause the respective imaging operations to more closely coincide with the image initiation signals, a process also referred to as “rephase.” However, this modification interferes with the placement of chevrons in any inter-image area in which rephase is occurring, because the image is positionally unstable during rephase and because the image initiation signal is asynchronous to the Master Clock. To minimize this instability, an additional zone is required on the photoreceptor belt for the formation of chevrons.
The efficiency of such electrophotographic printing machines is reduced because the requirement for a dedicated zone for the formation of chevrons limits the maximum number of image areas that can be formed in each cycle of the photoreceptor belt. The time required to adjust motor phase also limits the minimum inter-image area size, and further limits the efficiency of the electrophotographic printing machine.